Move the sides of the box around by clicking and dragging them until the box closely fits the dimensions of your head, then press space to start the head tracking. This requires your head to be the only moving object on camera. If your webcam refocuses, your virtual head may glitch. If your virtual head seems to lose tracking, just move your real head over to its general area, and things should work again.
The head tracking isn't perfect, but it's definitely functional (not to mention quite fast), and the cool projects that can be made with it are endless. Cloud head tracking anyone?
